NOTES ON THE DEVELOPMENT OF THE ARANEINA. 95 



The nuclei of the yolk have increased in number, but the 

 yolk, in other respects, retains its earlier characters. 



The next period in the development is that in v^hich the 

 body of the embryo gradually acquires the adult form. The 

 most important event which takes place during this period 

 is the development of the dorsal region of the embryo, which, 

 up to its commencement, is practically non-existent. As a 

 consequence of the development of the dorsal region, the 

 embryo, which has hitherto had what may be called a dorsal 

 flexure, gradually unrolls itself, and acquires a ventral 

 flexure. This change in the flexure of the embryo is in 

 appearance a rather complicated phenomenon, and has been 

 somewhat differently described by the two naturalists who 

 have studied it in recent times. 



For Claparede the prime cause of the change of flexure 

 is the translation dorsalwards of the limbs. He compares 

 the dorsal region of the embryo to the arc of a circle, the 

 two ends of which are united by a cord formed by the line 

 of insertion of the limbs. He points out that if you bring 

 the middle of the cord, so stretched between the two ends of 

 the arc, nearer to the summit of the arc, you necessarily 

 cause the two ends of the arch to approach each other, or, in 

 other words, if the insertion of the limbs is drawn up dor- 

 sally, the head and tail must approach each other 

 ventrally. 



Barrois takes quite a different view to that of Claparede, 

 which will perhaps be best understood if I quote a transla- 

 tion of his own words. He says: "At the period of the 

 last stage of the embryonic band (the stage represented in 

 PL IX, fig. 7, in the present paper) this latter completely 

 encircles the egg, and its posterior extremity nearly ap- 

 proaches the cephalic region. Finally, the germinal bands, 

 where they unite at the anal lobe (placed above on the 

 dorsal surface), form between them a very acute angle. 

 During the following stages one observes the anal segment 

 separate further and further from the cephalic region, and 

 approach nearer and nearer to the ventral region. This dis- 

 placement of the anal segment determines, in its turn, a 

 modification in the divergence of the anal bands ; the angle 

 which they form at their junction tends to become more 

 obtuse. The same processes continue regularly till the anal 

 segment comes to occupy the opposite extremity to the 

 cephalic region, a period at which the two germinal bands 

 are placed in the same plane and the two sides of the obtuse 

 angle end by meeting in a straight line. If we suppose a 

 continuation of the same phenomenon it is clear that the 



